."BIRDS. 



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The genera that were first created, belonged to the most 

 highly organised types of the class, whilst those which ex- 

 hibit a persistent form of some of the stages of embryonic 

 life, as the frogs and salamanders, did not appear until the 

 tertiary period. This fact, it will be remembered, exactly 

 accords with what has been observed in the stratigraphical 

 distribution of fossil fishes. 



CLASS III. BIRDS. 



THE class of birds is one of the most natural and best 

 defined in the animal series ; whether we take their ex- 

 ternal form and covering, or their internal organisation and 

 physiological peculiarities, as the basis of a definition. 



They are vertebrated animals, with warm blood, and a 

 double and complete circulation. They breathe the elastic 

 air, not only by lungs, but that life-giving fluid penetrates 

 abdominal air sacs, and even the bones themselves. The 

 anterior extremities are organised to form wings ; the body 

 is covered with feathers, and they are oviparous. 



Birds may be said to be to the vertebrated, what insects 

 are to the invertebrated classes. The structure of the 

 skeleton is very characteristic ; the bones are more compact, 

 white, dense, and brittle than those of mammalia, reptiles, 

 and fishes. They have thinner parietes, and their internal 

 cavities are proportionally larger, and they for the most part 

 contain air instead of marrow. 



Ossification proceeds to a greater extent in this than in 

 either of the other classes, consequently many bones which 

 remain separate through life in mammals and reptiles are 

 soldered together in birds. The typical form of the skeleton 

 is well preserved in all the different orders of this class. 

 The anterior extremities are adapted solely for flight, the 

 legs for support, and the head and neck are long and exten- 

 sively moveable for prehension. The trunk is fixed and solid, 

 to afford a firm surface for the attachment of the muscles 

 concerned in locomotion. The vertebrae of the neck and tail 

 are nearly the only bones of the spinal column that are 



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